K4Cu2+ Cu2+ Cl8 ∙2H2O: a novel non-centrosymmetric mixed-valent copper compound and its relation to minerals

Kornyakov I. V., Krivovichev S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 5. P. 103-114


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Language: English


Reddish-brown crystals of  K4Cu2+ Cu2+ Cl8 ∙2H2O were prepared by chemical transport reactions. The crystal structure was solved using single-crystal diffraction data (a = 9.0472(5), b = 11.5591(4), c = 9.1786(5) Å, β = 118.692(7)°, V = 842.01(9) Å3) and refined to R1 = = 0.039 for 4369 independent observed reflections. The crystal structure consists of the Cu2+ Cu +2Cl8 anionic chains extended along the a axis and linked through the K+ ions and H2O molecules. There are three Cu sites in the title compound. The Cu1 and Cu3 sites are occupied by monovalent Cu+ ions and coordinated tetrahedrally by four Cl atoms each with the Cu–Cl bond lengths in the range 2.309–2.441 Å. The (Cu1Cl4) and (Cu3Cl4) tetrahedra share Cl…Cl edge to form a (Cu2Cl6) dimer with relatively short Cu…Cu intermetallic distance of 2.585 Å. However, the theoretical analysis of the electron-density distribution shows the absence of bonding interaction between the adjacent Cu centers. The dimers are linked into 1D chains through the Cu2 atoms in an octahedral coordination. According to its coordination geometry and bond-valence calculations, the Cu2 site is occupied by Cu2+ ions. The K1 and K2 sites are coordinated by seven Cl atoms each to form (KCl7) capped trigonal prisms. In contrast, the K3 and K4 sites have a bicapped trigonal prismatic coordination by six Cl atoms and two H2O groups. The Cu2+ Cu+2 Cl8 chains are linked via K– Cl and K–H2O interactions as well as by H2O…Cl hydrogen bonds. The non-centrosymmetricity of the overall structure is the result of the shift of the adjacent chains relative to each other in the direction parallel to the a axis. The title compound is chemically close to avdoninite, K2Cu5Cl8(OH)4 · 2H2O, mitscherlichite, K2CuCl4 · 2H2O, and romanorlovite, K11Cu9Cl25(OH)4 2H2O. However, it differs from them in its mixed-valence characterThe title compound possesses neither unusually high or unusually low complexity and thus its formation as a secondary phase in fumaroles corresponds to the typical level of complexity observed in this geochemical environment.

Keywords: copper, mixed valence, non-centrosymmetric compounds, volcanic fumaroles, structural complexity